1. Technical Field
The present invention relates to a fuel cell formed by alternately stacking an electrolyte electrode assembly and separators. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Background Art
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as an electrolyte membrane. The electrolyte solid electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (electrolyte electrode assembly). Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell stack, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field (reactant gas flow field) is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between adjacent surfaces of the separators such that a coolant flows along the separators. Generally, fluid supply passages and fluid discharge passages extend through the fuel cell stack in the stacking direction of the separators. The fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field include plurality of flow grooves extending from the fluid supply passages to the fluid discharge passages, respectively. The flow grooves are straight grooves, or serpentine grooves.
However, if openings of the fluid supply passage or the fluid discharge passage are small for the plurality of flow grooves, it is required to provide buffers around the fluid supply passage and the discharge passage, respectively, so that a fluid such as the fuel gas, the oxygen-containing gas, or the coolant can flow along the flow grooves smoothly.
For example, a gas flow field plate of a fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 10-106594 is known. According to the disclosure of the Japanese Laid-Open Patent Publication No. 10-106594, as shown in FIG. 12, for example, a gas flow field plate 1 for forming a flow field of the oxygen-containing gas includes a groove member 2 made of carbon or metal. At an upper side of the gas flow field plate 1, an inlet manifold 3 for the oxygen-containing gas is provided. At a lower side of the gas flow field plate 1, an outlet manifold 4 for the oxygen-containing gas is provided.
The groove member 2 has an inlet side channel 5a connected to the inlet manifold 3, an outlet side channel 5b connected to the outlet manifold 4, and an intermediate channel 6 connected between the inlet side channel 5a and the outlet side channel 5b. A plurality of protrusions 7a are formed in the inlet side channel 5a and the outlet side channel 5b such that the inlet side channel 5a and the outlet side channel 5b have matrix patterns. The intermediate channel 6 has a serpentine pattern having a plurality of turn regions. The intermediate channel 6 includes a plurality of straight grooves 8 and channels 9 formed at the turn regions. A plurality of protrusions 7b are formed in the channels 9 such that the channels 9 have matrix patterns.
In the gas flow field plate 1 constructed as described above, the inlet side channel 5a and the outlet side channel 5b function as buffers. Thus, the contact area between the supplied gas and the electrode is large, and the supplied gas can move freely. Further, in the intermediate channel 6, the reactant gas flows uniformly at high speed through the plurality of straight grooves 8.
In the gas flow field plate 1, practically, a plurality of serpentine passages 1a extending from the inlet manifold 3 to the outlet manifold 4 are formed. In the plurality of the straight grooves 8, the respective passages 1a have substantially the same length. Thus, the flow resistance tends to be constant in each of the passages 1a. 
However, in the inlet side channel 5a and the outlet side channel 5b which are formed in the matrix patterns by the plurality of protrusions 7a, the passages 1a from the inlet manifold 3 and the outlet manifold 4 to the respective straight grooves 8 have different lengths. Therefore, the flow resistance varies in the inlet side channel 5a and the outlet side flow channels 5b, and thus, it is not possible to supply the reactant gas uniformly over the entire surface of the electrode. Consequently, the reactant gas is not distributed desirably.
Likewise, in the matrix pattern channels 9 formed by the plurality of the protrusion 7b, when reactant gas flows out of the respective straight grooves 8, turns back in the matrix pattern channels 9, and flows into the respective straight grooves 8, since the flow passages 1a have different lengths, the reactant gas are not distributed uniformly. Thus, the reactant gas is not supplied uniformly over the entire surface of the electrode. Thus, the desired power generation performance can not be maintained.
Further, a coolant flow field may be formed on the back surface of the gas flow field plate 1 for supplying a coolant along the surface of the gas flow field plate 1. In this case, for example, an inlet manifold 3a of the coolant is provided adjacent to the inlet manifold 3, and an outlet manifold 4b of the coolant is provided adjacent to the outlet manifold 4. The inlet side channel 5a and the outlet side channel 5b may function as buffers for supplying the coolant to the coolant flow field, and discharging the coolant from the coolant flow field on the back surface of the gas flow field plate 1.
However, the inlet side channel 5a and the outlet side channel 5b as the buffers have a square shape or a rectangular shape. Therefore, the inlet manifolds 3, 3a, and the outlet manifolds 4, 4a can not be provided in a small space on the surfaces of the gas flow field plates efficiently. Therefore, the area of the gas flow field pate 1 which is not used for reaction increases, and the output density per unit area is lowered. Consequently, the gas flow field plate 1 itself has a considerably large size.